1 male to an enclosure unless its a REALLY big enclosure (havent seen any dimensions but probably 7-8foot or more) - females seem to get on fine. Make sure females have a deep, dark place to lay eggs, or they may become eggbound.
In the wild they live in loose hierachical groups with one dominant male (or "Tyrant" male) defending the foods and females from lesser males (although Im pretty sure these lesser males may still live in the same area - they will just be domineered with regards to mates and resources). I have had mine in pairs and trios, and seen people keeping them in larger groups than this; I dont know though whether or not its detrimental to a chuckwalla to house it alone in light of this relatively social behaviour. Certainly I know people that have kept them alone with no apparent ill effects...so to answer your question, I dont know the answer to whether or not its better for them one way or another I havent had any problems with mine being housed together in pairs or trios; they seem quite happy to interact with one another.
5x2x2 is a great size for one, two or three! Put plenty of flat rocks piled up to make hiding and climbing areas; chucks like to be "wedged" in someplace when they are resting or frightened. Theyre surprisingly strong though so make sure that they cant move the rocks and accidentally get squashed!